Paper mills coat moving paper webs with coating compositions to achieve various desired properties in the finished paper (e.g. printability, moisture resistance, and the like).
The techniques for applying coating compositions to paper vary. One of the more common and simplest methods of application of coating compositions to moving paper webs in paper mills is by the use of blade coaters, such as the trailing blade coater.
When blade coating was first developed into a practical method of applying aqueous coatings to paper and paper-board, it soon became apparent that scratches in the finished product were a major obstacle to be overcome if the blade coater was to become a commercial success. While many factors contribute to blade scratches, one major factor within the control of the paper mill was coating solids. In most cases, reducing the percent solids at which the coating was applied reduced the frequency of blade scratches and made the compositions less viscous and easier to handle and apply.
The design of a suitable coating composition is often difficult because, in many instances, the desired end properties in the coated paper appear to be almost mutually exclusive. By this it is meant that as one desired property is improved by changing the coating compositions or other coating parameters, the other desired property is diminished. Consequently, coating compositions in commercial use are the product of compromise.
In spite of this, the use of liquid coating compositions in blade coating apparatus has become one of the most widely used coating techniques. However, when higher quality paper products are desired special processes, compositions and apparatus are often used in combination (e.g. the use of highly polished chromium-plated drums to impart high gloss and surface smoothness to coated paper). These special techniques are often effective, but tend to be costly and often require the use of apparatus other than the blade coating apparatus used for making many common grades of coated paper.
In view of the widespread use of blade coating apparatus, various attempts have been made to utilize such equipment in combination with other techniques to achieve certain desired properties in coated paper such as porosity, smoothness, and the like. Such techniques include: (1) the use of blade coating techniques followed by brushing or supercalendering (e.g. supercalendering through 5-8 nips under pressure); (2) increasing the coating weight; and (3) using multiple coatings. However, each of these procedures has its own disadvantages (e.g. supercalendering smoothes the paper while densifying or compacting it (which is undesirable for some significant commercial purposes).
Accordingly, there is a need for means to be devised whereby blade coating apparatus (e.g. inverted blade coaters) can be used to produce improved coated paper (e.g. improved as to smoothness, ease of finishing, porosity, and the like). Such a procedure would eliminate the need for multiple types of coating apparatus and the related investment. Further, it would reduce the cost of making such products. Desirably, such a procedure will be effective at relatively low coating weights and will produce useful improvements in a single pass (i.e. avoid the use of multiple coatings on each side of the paper web). Further, such a procedure should utilize a substantial amount of clay pigment (as a percent of total pigment).